1. Field of the Invention
This invention relates to a method for preparing a photothermographic material, especially suited for the manufacture of printing plates.
2. Prior Art
Photothermographic materials which are processed by a photothermographic process to form photographic images are disclosed, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075, D. Morgan and B. Shely, "Thermally Processed Silver Systems" in "Imaging Processes and Materials," Neblette, 8th Ed., Sturge, V. Walworth and A. Shepp Ed., page 2, 1969.
These photothermographic materials generally contain a reducible silver source (e.g., organic silver salt), a catalytic amount of a photocatalyst (e.g., silver halide), a toner for controlling the tonality of silver, and a reducing agent, typically dispersed in a binder matrix. Photothermographic materials are stable at room temperature. When they are heated at an elevated temperature (e.g., 80.degree. C. or higher) after exposure, a redox reaction takes place between the reducible silver source (functioning as an oxidizing agent) and the reducing agent to form silver. This redox reaction is promoted by the catalysis of a latent image produced by exposure. Silver formed by reaction of the organic silver salt in exposed regions provides black images in contrast to unexposed regions, eventually forming an image.
Such photothermographic materials have been used as microphotographic and radiographic photosensitive materials. However, only a few have been used as a graphic printing photosensitive material because the image quality is poor for the printing purpose as demonstrated by low maximum density (Dmax) and soft gradation.
With the recent advance of lasers and light-emitting diodes, scanners and image setters having an oscillation wavelength of 600 to 800 nm find widespread use. There is a strong desire to have a high contrast photosensitive material which has so high sensitivity and Dmax that it may comply with such output devices. Also a need for easy and dry processing is increasing.
U.S. Pat No. 5,464,738 describes that high contrast images are obtainable using sulfonyl hydrazide as a reducing agent for dry silver. However, development does not take place unless the developing temperature is raised as high as 136.degree. C. to 142.degree. C.
Dimensional stability is one of the most important attributes required for photosensitive materials for use as printing plates. As the developing temperature rises, plastic film used as the support undergoes thermal shrinkage, incurring dimensional changes. Such dimensional changes are undesirable because color shift and noise associated with white or black lines appear in printed matter. It is generally desired to restrict a dimensional change to 0.01% or less before and after development.
An attempt was made to lower the heat developing temperature. The attempt succeeded in lowering the temperature, but to about 110.degree. C. At such temperature, supports of polyethylene terephthalate will experience a thermal shrinkage of about 0.1%. Dimensional stability is still insufficient.
On the other hand, it is contemplated to form the support from a material which experiences a minimal dimensional change at elevated temperatures. Such supports are of polycarbonate, polysulfone, polyarylates, polyether sulfone, polyethylene naphthalate, and polyimides. Inter alia, polycarbonate is recommended as a printing plate film support because it is improved in transparency and light transmission in the UV region. The polycarbonate, however, has never been used as the support of photothermographic material used for the above-mentioned purpose because it is readily soluble in organic solvents, especially ketones such as acetone and methyl ethyl ketone. Such solubility is undesirable because photothermographic material is generally prepared by adding an organic solvent solution of a reducing agent to an organic solvent dispersion of organic silver salt and silver halide and coating the resulting solution to a film support. Methyl ethyl ketone is the most popular organic solvent.
The coating step using an organic solvent is one of the factors causing the reduced production yield of the photosensitive material manufacturing process because it is difficult to increase the coating rate and to simultaneously coat multiple layers.